1. Field of the Invention
The present invention relates to a color image forming apparatus and, more particularly, to a color image forming apparatus for forming a color image by a digital system.
2. Description of the Prior Art
Demands for color copies bearing much information are recently increasing in accordance with the wide spread of the color image devices of computer color CRT display systems or videotex terminal system and the coloration of documents and printed matters in firms and offices.
From this background, a variety of methods and apparatus for forming multi-color images have been proposed.
In Japanese Patent Laid-Open Nos. 52-106743, 56-144452, 58-79261 and 61-170754, for example, there is disclosed a method by which a plurality of latent image forming means and developing means are arranged around a rotating drum-shaped photosensitive member so that visible images of different colors are overlapped on the drum-shaped photosensitive member by repeating the latent image formations and developments and are transferred altogether to a recording paper.
The resists of the toner images on an image retainer using a plurality of image pickup elements according to the above-specified system are far better improved than those of the color reproducing system using a transfer drum according to the prior art but are still troubled by serious problems such as the color drifts or Moire due to the color image pickup elements.
These problems are caused by the fact that dislocations of pixels are unavoidable in a color contact sensor type or a reduced focal type of prism separation.
In the former type, color correction is accomplished from information having a dislocation of one third pixel because B, G and R filters are disposed alternately. In the latter type, three CCDs have to be positioned in an accuracy of about 0.2 pixels (wherein one pixel is about 10 microns), which is considerably difficult. Another problem of chromatic aberration also arises. Because of the requirement for the B, G and R information, moreover, it is difficult to speed up the CCD drive, and the processing of images and the provision of sensors raise the cost.
In Japanese Patent Laid-Open Nos. 60-76766, 60-95456 and 61-170754, on the other hand, there is also disclosed a method by which one latent image forming means and a plurality of developing means are arranged around a rotating drum-shaped photosensitive member so that a latent image of one color is formed and developed for each rotation of the photosensitive member and so that a multi-color visible image is formed on the photosensitive member by a plurality of rotations of the photosensitive member and are transferred altogether to a recording paper.
Since the latent image forming means is singular, according to this method, the system can be more small-sized than the former method. Since, moreover, the latent image forming means is commonly used, the latter method is advantageous in retaining the latent images on the resists.
As a color image pickup system, there is also known a method by which filters and light sources are switched.
These color image inputting methods have been revealed to be effective for preventing the Moire partly because they are basically reluctant to be followed by the color drifts and partly because they can make use of solid-state monochromatic image pickup elements of high resolution.
This implies that the deterioration in the images is highly caused by the mistaken color reproduction errors of the color judgement due to the dislocations of pixels and by the Moire.
For superposition of the toner images, on the other hand, it has apparently become necessary to use a method for compensating the influences of the foregoing toner image retaining face, in place of the color correction method of B, G and R signals.